Background: Conventional Methods for Measuring Water Content
A determination of water content in crude oil is required to measure accurately net volumes of actual oil in sales, taxation, exchanges, and custody transfers. The water content of crude oil is also significant because it can cause corrosion of equipment and problems in processing. Thus, various methods have been developed for measuring the water content of crude oil.
Background: Karl Fischer Titration Method
In 1935, the German scientist, Karl Fischer, developed a titrimetric determination of water content using a reagent that contained iodine, sulphur dioxide, anhydrous pyridine and anhydrous methanol. This method can be subdivided into two main techniques: volumetric titration and coulometric titration.
The volumetric technique involves dissolving the sample in a suitable solvent and adding measured quantities of a reagent containing iodine until an end point is reached. This end point is determined potentiometrically using a platinum electrode. When all of the water has reacted, the platinum measuring indicator electrode will electronically instruct the burette to stop dispensing. The volume of KF reagent dispensed is recorded. Based on the concentration of iodine in the KF reagent, the amount of water present is then calculated.
However, even with automatic or semi-automatic instruments commercially available, there are certain problems associated with this technique. These problems include long analysis time, required reagent calibration, and high reagent consumption rate.
In the coulometric technique developed by Meyer and Boyd in 1959, the sample is introduced into a mixture of pyridine/methanol that contains iodide ions and sulphur dioxide. The electrode system consists of an anode and cathode platinum electrodes that conduct electricity through the cell. Iodine is generated at the anode and reacts with any water present. The production of iodine is directly proportional to the amount of electricity according to Faraday's Law as shown in the equation:2I−−2e→I2.
According to the stoichiometry of the reaction, 1 mole of iodine will react with 1 mole of water, and combining this with coulometry, 1 milligram of water is equivalent to 10.71 coulombs of electricity. Therefore, it is possible to directly determine the amount of water present in a sample by measuring the electrolysis current in couloumbs. The platinum indicating electrode voltametrically senses the presence of water and continues to generate iodine until all the water in the sample has been reacted.
From this titration, the on board microprocessor calculates the total amount of current consumed in completing the titration and the time to completion in seconds. Based on the relationship between coulombs and iodine, the exact amount of iodine generated is recorded. Since water reacts in the 1:1 ratio with iodine, the amount of water can be calculated.
Although the original Karl Fischer reagent contained pyridine, most reagent manufacturers now use other amines such as imidazol.
Karl Fischer titration is one of the most widely used techniques for measuring the water content in a large range of samples. However, it has limits that affect its usefulness for on-site detection of moisture in petroleum samples. For example, it utilizes hazardous reagents that require the user to exercise care in the storing, handling, and disposing of the reagents. The small sample size utilized by the techniques causes errors. Also, the technique cannot measure water percentages over 1% accurately.
(Please see Manual of Petroleum Measurement Standards, Chapter 10.7—Standard Test Method for Water in Crude Oils by Potentiometric Karl Fischer Titration and Chapter 10.9—Determination of Water in Crude Oils Coulometric Karl Fischer Titration for the complete protocols which are hereby incorporated by reference.)
Background: Centrifuge Method
In the standard method for determining the water content in crude oil by centrifuge, equal volumes of a crude oil sample and water saturated toluene are placed into two cone-shaped centrifuge tubes. The tubes are then corked and placed into a centrifuge. The tubes are then spun for 10 minutes at a minimum relative centrifugal force of 600 calculated from the following equation:rpm=1335√{square root over (rcf/d)}where:                rcf=relative centrifugal force and        d=diameter of swing measured between tips of opposite tubes when in rotating position, mm.        
Immediately after the centrifuge comes to rest following the spin, the combined volume of water and sediment at the bottom of each tube is read and recorded. The spin is then repeated until the combined volume of water and sediment remains constant for two consecutive spins. The final volume of water is then recorded for each tube.
The standard method for determining the water content in crude oil by centrifuge is not entirely satisfactory. The amount of water detected is almost always lower than the actual water content. Therefore, when a high accurate value is required, another method must be used. This method also requires hazardous solvents, and has very poor accuracy and reproducibility.
(Please see Manual of Petroleum Measurement Standards, Chapter 10.3—Standard Test Method for Water and Sediment in Crude Oil by the Centrifuge Method (Laboratory Procedure) for the complete protocol which is hereby incorporated by reference.)
Background: Distillation Method
In the standard test for determining the water content in crude oil by distillation, the crude oil sample is heated under reflux conditions with a water immiscible solvent that co-distills with the water in the sample. The condensed solvent and water are continuously separated in a trap wherein the water settles in the graduated section of the trap, and the solvent returns to the distillation flask. The amount of water can then be determined on a volume or a mass basis.
The precision of this method can be affected by water droplets adhering to surfaces in the apparatus and, therefore, not settling into the water trap to be measured. To minimize this problem, all apparatus must be chemically cleaned at least daily to remove surface films and debris that hinder the free drainage of water in the apparatus.
The drawbacks to this method include, for example, the fact that it utilizes hazardous solvents and produces hazardous vapors. This method also takes 2 to 3 hours to complete, and as with most distillation techniques, the accuracy and precision of the results will depend upon the skill of the technician performing the distillation.
(Please see Manual of Petroleum Measurement Standards, Chapter 10.2—Standard Test Method for Water in Crude Oil Distillation for the complete protocol which is hereby incorporated by reference.)
Background: Zeolite Molecular Sieves
Molecular sieves, as used in this specification, include any material that can effectively be used to sequester or restrain or retain molecules in a material, such as, but not limited to, water molecules in a non-aqueous liquid, whether by physical capture within a crystalline structure, absorptive properties, adsorption, hydrogen bonding, or other means.
One class of molecular sieves includes crystalline, hydrated metal aluminosilicates. The commercially important types of molecular sieves are synthetically made, but their structure is similar enough to naturally occurring minerals to be classified as zeolites. Although the crystal structures of some of the molecular sieves are quite different, their absorbent property derives from their crystalline structure.
The crystalline metal aluminosilicate molecular sieves have a simple polyhedra arrangement. Each polyhedron is a three-dimensional array of (Si, AlO4) tetrahedral. The tetrahedra are formed by four oxygen atoms surrounding a silicon or aluminum atom. Each oxygen atom has two negative charges and each silicon atom has four positive charges. This structure permits a net sharing arrangement, building a tetrahedron uniformly in four directions. The trivalency of aluminum causes the alumina tetrahedron to be negatively charged, requiring an additional cation to balance the system. Thus, the final structure has sodium, potassium, or calcium cations in the network. These “charge balancing” cations are the exchangeable ions of the zeolite structure.
Zeolites, one class of molecular sieves, exhibit electrical conductivity of an ionic type due to the migration of cations through the channel structure. The ability of the cations to carry a current depends upon their ionic mobility, charge, size, and location in the structure. The addition of water molecules to a dehydrated zeolite structure produces a pronounced change in the electrical conductivity of the zeolite. The conductivity of the zeolite increases with the amount of water present. The activation energy for conduction decreases with increasing adsorption of water. The influence of water is different for different zeolites. In some cases, the activation energy for conduction in a zeolite containing divalent ions is approximately twice that of a zeolite containing univalent ions.
When formed, this crystalline network is full of water, but with moderate heating, the moisture can be driven from the cavities without changing the crystalline structure—leaving countless cavities with their tremendous combined surface area and pore volume available for adsorption of water or other materials.
With their large surface area and pore volume, molecular sieves then can perform virtually all the adsorption duties presently carried out by other absorbents. In addition, molecular sieves allow for a new dimension in process control because the pores of the crystalline network are uniform rather varied. Therefore, molecular sieves are able to differentiate molecules on the basis of molecular size and configuration.
Hence, molecular sieves utilize two adsorption mechanisms. They exhibit the capillary condensation phenomenon as a result of their large surface area and pore volume, and their polar surfaces have an electrostatic attraction for polar molecules such as water. This allows molecular sieves to be stronger absorbents than silica gel or alumina.
Another advantage to molecular sieves is that they can be packaged in foil-sealed bags to prevent moisture adsorption. This allows them to have long term stability and makes them easy to use. Also, the measured quantity of molecular sieves can be accurately controlled.
Although this application refers to the adsorptive properties and activities of molecular sieves, it understood that a certain amount of absorption also takes place. Therefore, for the sake of simplicity, references to the adsorptive properties and activities of molecular sieves are intended to include any absorptive properties and activities as well.
Background: The “Load-Pulled” Effect
It is well known to electrical engineers generally (and particularly to microwave engineers) that the frequency of an RF (radio frequency) oscillator can be “pulled” (i.e. shifted from the frequency of oscillation which would be seen if the oscillator were coupled to an ideal impedance-matched pure resistance), if the oscillator sees an impedance which is different from the ideal matched impedance. Thus, a varying load impedance may cause the oscillator frequency to shift.
The present application sets forth various innovative methods and systems which take advantage of this effect. In one class of embodiments, an unbuffered RF oscillator is loaded by an electromagnetic propagation structure which is electromagnetically coupled, by proximity, to a material for which real time monitoring is desired. The net complex impedance seen by the oscillator will vary as the characteristics of the material in the electromagnetic propagation structure vary. As this complex impedance changes, the oscillator frequency will vary. Thus, the frequency variation (which can easily be measured) can reflect changes in density (due to bonding changes, addition of additional molecular chains, etc.), ionic content, dielectric constant, or microwave loss characteristics of the medium under study. These changes will “pull” the resonant frequency of the oscillator system. Changes in the medium's magnetic permeability will also tend to cause a frequency change, since the propagation of the RF energy is an electromagnetic process which is coupled to both electric fields and magnetic fields within the transmission line.
Background: Aluminum Oxide for Moisture Adsorption
The use of aluminum oxide for moisture adsorption is well known in the industry. The surface attracts and retains water molecules by association with the bonds. Since this is a weak attraction there is a point at which the absorption and desorption reaches an equilibrium with the surrounding moisture content. Moisture measurements have been made with capacitance measurements using a very thin aluminum oxide surface with imbedded electrodes. When the water is absorbed the capacitance changes and therefore a measurement is made. This surface must be thin in order to allow the water molecules to accumulate in a region where the electrical field is present.
For further background and information on load pulled systems, the reader is referred to U.S. Pat. No. 6,630,883 to Scott, which is hereby incorporated by reference.
Density Independent Moisture Analyzer
The present application describes systems and methods for the on-site determination of water content in crude oil.
The present innovations include, in one embodiment, placing a material to be tested in a container or package of a molecular sieve material. This container is then placed in a microwave measurement system (or other scattering parameter measuring system). By measuring the effects of the sample on scattering parameters, the sample can be characterized.
For example, the innovations can detect an amount of water in another material, such as crude oil, by placing a crude oil sample in the container with the molecular sieves, and by measuring the effects on the scattering parameters, an estimate of the water content of the oil can be determined.
Hence, the disclosed innovations provide a simple approach to measuring the moisture content in crude oil that is extremely fast, accurate, and reproducible without the use of hazardous chemicals.